The petroleum source is beginning to exhausted, and at the same time, environmental problems such as global warming due to the consumption of fossil fuel have increasingly become serious. Thus, a fuel cell receives attention as a clean power source for electric motors that is not accompanied with the generation of carbon dioxide. The above fuel cell has been widely developed, and some fuel cells have become commercially practical. When the above fuel cell is mounted in vehicles and the like, a polymer electrolyte fuel cell comprising a polymer electrolyte membrane is preferably used because it easily provides a high voltage and a large electric current.
As an electrode structure used for the above polymer electrolyte fuel cell, there has been known an electrode structure, which comprises a pair of electrode catalyst layers comprising a catalyst such as platinum supported by a catalyst carrier such as carbon black that is formed by integrating by an ion conducting polymer binder, a polymer electrolyte membrane capable of conducting ions sandwiched between the electrode catalyst layers, and a backing layer laminated on each of the electrode catalyst layers. When a separator acting also as a gas passage is further laminated on each of the electrode catalyst layers, the above electrode structure constitutes a polymer electrolyte fuel cell.
In the above polymer electrolyte fuel cell, one electrode catalyst layer is defined as a fuel electrode, and the other electrode catalyst layer is defined as an oxygen electrode. Now, reducing gas such as hydrogen or methanol is introduced into the fuel electrode through the above backing layer, whereas oxidizing gas such as air or oxygen is introduced into the oxygen electrode through the above backing layer. By this action, on the above fuel electrode side, protons are generated from the above reducing gas by the action of a catalyst contained in the above electrode catalyst layer. Then, the protons transfer to the electrode catalyst layer on the above oxygen electrode side through the above polymer electrolyte membrane. Thereafter, the protons are reacted with the above oxidizing gas introduced into the oxygen electrode by the action of the above catalyst contained in the electrode catalyst layer on the above oxygen electrode side, so as to generate water. Thus, the above fuel electrode is connected to the above oxygen electrode through using a conductor, so as to obtain electric current.
Previously, in the above electrode structures, a perfluoroalkylene sulfonic acid polymer (e.g., Nafion (trade name) from DuPont) has been widely used for the above polymer electrolyte membrane. The perfluoroalkylene sulfonic acid polymer is sulfonated, and accordingly it has an excellent proton conductivity. The compound also has a chemical resistance as a fluorocarbon resin. However, the compound has a problem in that it is extremely expensive.
Thus, the use of a relatively inexpensive ion conducting material instead of the perfluoroalkylene sulfonic acid polymer has been under study for constituting an electrode structure for a polymer electrolyte fuel cell. An example of the above inexpensive ion conducting material may include a hydrocarbon-based polymer.
However, the hydrocarbon-based polymer is poor in toughness, and so it is difficult to use it as a polymer electrolyte membrane to constitute the above electrode structure. In order to improve the toughness of the hydrocarbon-based polymer, for example, methods such as introducing a bending group into the main chain of the hydrocarbon-based polymer, or reducing the ion exchange capacity are being considered.
However, when the hydrocarbon-based polymer whose toughness is improved as described above is used for the polymer electrolyte membrane of the electrode structure, there is an inconvenience in that it is difficult to obtain a sufficient power generation efficiency. In addition, the hydrocarbon-based polymer is inconvenient in that it has a low oxidation resistance and it deteriorates rapidly.